1. Field of the Invention
The present invention relates to a semiconductor device and a method for making the same, and in particular to a semiconductor device including one or more semiconductor layers formed as one or more functional layers on a semiconductor substrate and composed of a compound semiconductor and a method for making the same. The present invention is extensively employed for a functional device including an electronic device such as a diode and transistor, an optical device such as an LED, LD and optical waveguide, and a sensor.
2. Description of the Related Art
Generally, the technique for making a semiconductor layer or layers on a semiconductor substrate is broadly classified into a planer technique principally using the diffusion, ion implantation, oxidation, etc., and an epitaxial growth technique principally using a liquid phase epitaxy (LPE) and vapor phase epitaxy (VPE and CVD). In the formation of a compound semiconductor, in particular, the epitaxial growth technique is employed as a technique for forming semiconductor crystal or mixed crystal layers of a different composition on a semiconductor substrate. This technique has been extensively employed to fabricate an electronic device such as a hetero-bipolar transistor (HBT) and high electron mobility transistor (HEMT), and an optical device such as a light emitting diode (LED), laser diode (LD). photodiode (PD) and optical waveguide, all these devices requiring a hetero-junction structure of a different composition.
The epitaxial growth technique includes, in addition to a liquid phase epitaxy (LPE) method and vapor phase epitaxy (VPE) method using a halogen transport, a metal-organic vapor phase epitaxy (MOVPE) and a molecular beam epitaxy (MBE) method (including an MOMBE method), as capable of a greater material selectivity as well as ready making of a fine structure, an atomic layer epitaxy (ALE) method, a photochemical VPE method using light, as a decomposition energy for a material, in place of heat, and so on.
The conventional semiconductor process technique can readily deposit layers of a different composition on a semiconductor substrate in a direction vertical to the substrate surface, but it has been difficult to deposit layers having a different composition area in a direction parallel to the substrate surface.
Forming a simple optical waveguide on a semiconductor will be explained below by way of example.
In order to provide an optical waveguide over a semiconductor substrate, a semiconductor area acting as the optical waveguide is formed such that it is surrounded with a clad area of a lower refractive index than that of the semiconductor area. FIGS. 1A to 1C are perspective views showing various forms of an optical waveguide formed on a semiconductor substrate. FIG. 1A is an example in which an optical conducting layer 2 is formed on a substrate 1 such that it is surrounded with cladding layers 3 in a direction parallel to the surface of the substrate. FIG. 1B is an example in which an optical conducting layer 2 is formed on a substrate 1 such that it is surrounded with upper and lower cladding layers 3, in a stacking fashion, in a direction vertical to the surface of the substrate 1. FIG. 1C is an example in which an optical conducting layer 2 and cladding layers 3' are formed as one unit on the surface of a substrate 1 in a direction parallel to the surface of the substrate in which case the unit is sandwiched with cladding layers 3 in that vertical direction except for opposite end faces of the optical conducting layer 2. The structure as shown in FIG. 1C allows light to be conducted to the inner region of the optical conducting layer 2. It will be readily understood that the structure of FIG. 1C is composed of a combination of the structures of FIGS. 1A and 1B.
Usually in the case where an optical waveguide is provided on the substrate with the use of a semiconductor crystal, the following conditions have to be met:
(a) The refractive index of the optical conducting layer, upon being compared with that of the cladding layer, is great enough to confine light therein.
(b) Both the optical conducting layer and cladding layer can be formed as a single crystal on the semiconductor substrate.
As a method for satisfying these conditions (a) and (b), use has usually been made of a mixed crystal of, for example, III - V or II - VI compound semiconductors. This is because the lattice constant and band gap energy (Eg) can be controlled by varying the composition of constituent elements of which the mixed crystal is formed and because the refractive index has a correlation to the band gap energy, that is, an increase in the band gap energy (Eg) leads to a decrease in the refractive index.
The method for the making of a structure as shown in FIGS. 1A and 1B will be explained below in more detail.
The structure of FIG. 1B can readily be made by an epitaxial growth technique for semiconductors, that is, a cladding layer 3, optical conducting layer 2 and cladding layer 3 are sequentially grown over a semiconductor substrate 1 by an epitaxial growth method, such as a liquid phase epitaxy (LPE) or vapor phase epitaxy (VPE). If, at this time, the thickness of these layers has to be exactly controlled on the order of a few .mu.m or below, it is advantageous to employ a molecular beam epitaxy (MBE) or a metalorganic vapor phase epitaxy (MOVPE) method.
On the other hand, the structure of FIG. 1A is made as shown in FIG. 2. That is, an optical conducting layer 2 is grown as a crystal on a semiconductor substrate 1, by the epitaxial growth technique, as shown in FIG. 2A. Then a mask layer, such as an SiO.sub.2 film, is patterned, by a photoetching process (PEP), on the optical conducting layer 2 as shown in FIG. 2B and the optical conducting layer 2 is removed, by the etching process, at an area not covered with the mask 4 as shown in FIG. 2C. Cladding layers 3 are selectively grown, by the epitaxial growth technique, on the resultant exposed surface of the semiconductor substrate 1 as shown in FIG. 2C and, finally, the mask 4 on the optical conducting layer 2 is removed to expose the latter as shown in FIG. 2E.
However, the problems as will be set forth below arise in the making of the optical waveguide shown in FIG. 2A. First, the exposed side surfaces of the optical conducting layer 2 are injured or "side etched", by the etching process, during the making step as shown in FIG. 2C, or the exposed surfaces of the optical conducting layer are oxidized depending upon the material used at that process. It is, therefore, difficult to obtain a pure, exposed crystal face. Second, it is usually difficult to obtain flat boundaries relative to the side surfaces of the optical conducting layer 2 during the formation of the cladding layers relative to the optical conducting layer 2 by the selective epitaxial growth as shown in FIG. 2D, that is, to obtain buried cladding layers 3 of a flat boundary relative to the layer 2. To put it in another way, discontinuous surfaces are liable to be produced between the optical conducting layer and the cladding layer. For the reasons as set out above, such discontinuous surfaces and defects occur at the boundaries between the optical conducting layer and the cladding layers 3, thus causing the absorption or scattering of light at the boundaries of the optical waveguide. Third, it is generally difficult to form an epitaxial layer, by a selective epitaxial growth method, on the surface of the mask and remaining surface of the resultant structure, as shown in FIG. 2D, and it is possible to perform such a selective epitaxial growth under the specific conditions only.
Although forming a semiconductor layer of a different crystal composition in the direction parallel to the surface of the substrate has thus far bee explained, as a typical example (FIG. 1A), in connection with the optical waveguide, it is very difficult to employ a conventional process in the formation of such a semiconductor layer.
It is to be noted that the structure of the optical waveguide as shown in FIG. 1C is obtained by a combination of the process methods shown in FIGS. 1A and 1B.
For a semiconductor device in which semiconductor layers are formed as functional layers on a semiconductor substrate, in general, it is easier to sequentially form layers of different compositions on the semiconductor substrate in stacked fashion as shown in FIG. 1B, but it becomes very difficult to form such layers of different crystal compositions in a lateral direction, that is, in a direction parallel to a substrate surface as shown in FIG. 1A. The aforementioned wafer process often includes, for example, a selective etching and selective epitaxial growth process. Further, the semiconductor layers as stacked in a complex process over the semiconductor substrate contain discontinuous surfaces or crystal defects which are liable to occur at the boundaries between these layers of different crystal compositions. This is a cause for the degraded characteristics of a semiconductor device obtained.